1. Field
This invention relates to metal and ceramic casting methods. Specifically, it relates to a combination vacuum assist centrifugal casting method and apparatus.
2. State of the Art
A number of metal and ceramic casting methods are known. The lost wax replacement casting method involves: 1) making a model of the object to be cast, 2) spruing the model to a wax tree at one or more points to form a path for the molten metal to flow; 3) mixing and pouring the investment into a mold to surrounding the model, 4) burning off the wax model and sprue to form a mold in a casting flask, and 5) and filling the resulting mold with metal.
Generally, the gold, silver, bronze, etc. to be cast is melted directly (either in a centrifugal machine or in a separate pouring crucible) with a hand torch. Another method is to melt the metal in a pouring crucible in a gas or high frequency induction casting furnace, and then pour the metal into the casting flask.
To minimize voids and porosity in the casting caused by trapped gases in the mold, two types of methods for casting metals utilizing the lost wax replacement method were developed: centrifugal casting, and vacuum assist casting.
Centrifugal casting was initially developed by dentists where small casts were required. Centrifugal casting utilizes a centrifuge to hurl molten metal to fill the voids of the pattern mold of a casting flask left after the wax has been burned off. Centrifugal casting apparatus typically have a centrifuge operably associated with a spinning arm on which a casting flask is mounted toward the end of the arm with the opening of the mold held in proximity to the opening of a casting crucible; see Hellebrand et al, U.S. Pat. No. 4,726,413, Zu Der Patentshcrift Nr. 133337; Goodrich et al, U.S. Pat. No. 4,134,445; as examples of centrifugal casting methods.
There are two basic methods for creating the wax model: carving or cutting solid wax into the form, and the building of a model from scratch, or a combination of the two. The model is formed from wax which comes in various sizes, shapes, and textures. Wax may be obtained in wire form, sheets, solid block, preformed and semi-preformed wax. Wax is color coded. This code varies with the manufacturer, so it cannot be depended upon when using wax from various manufacturers. The wax is worked by filing, drilling, cutting, adding and removing, using regular bench tools, plus carving tools that have been borrowed from the dental profession. These dental type tools are warmed over a flame to work the wax. Rings are formed on a mandrel which gives the correct desired size to the model, so the finished ring will be produced in a predetermined size without the problem of sizing later.
The method of attaching the sprue wax to the model at one or more points to form a path for the molten metal to flow without leaving a void in any of the distant parts is known as spruing the model.
The following must be completed before you proceed to mix the invest slurry. The model must be correctly and securely sprued to the appropriate size sprue-base and a clean flask to fit this base must be handy. The model must have also been painted with a surface tension reducer which has had time to air-dry. Other methods, such as Lassow et al., U.S. Pat. No. 4,703,806 utilize a mold facecoat and core coating system for investment casting of reactive metals.
When mixing, the powder is added to water at room temperature, approximately 70-75 degrees. If the water is too cold, it will delay the hardening process. If it is too warm, it will speed up this process. If available, the manufacturer's quantities and ratios for mixing should be followed.
The following table will give general information for the common and most used size flasks:
______________________________________ CAPACITY OF FLASKS Water Powder Flask Size Ounces Ounces Height (in.) Diameter (in) cc. Avoir. Grams Avoir. ______________________________________ 11/2 .times. 11/4 20 7/10 50 13/4 15/8 .times. 13/4 40 1 2/5 100 31/2 23/8 .times. 21/2 90 3 1/5 225 8 21/2 .times. 31/2 180 6 3/10 450 153/4 ______________________________________
When painting a model, the investment is pushed in front of the brush rather than a dabbing action. This helps prevent trapped air from being caught in the crevices of the model. When pouring investment to surround the model, the investment is poured down the side of the flask rather than on top of the model again to avoid trapping air and causing an imperfect mold.
After the investment has been mixed, the wax model sprued and attached to its base, and the model painted with investment; the flask is then fitted to the base and readied to pour investment. The flask is then filled with investment completely surrounding the wax model.
The next step is to purge the flask of any trapped air. This is done on a vibrator or with the aid of a vacuum machine. Whichever method is used, the flask is left on the unit until escaping air bubbles are no longer visible. The invested flask is then set aside to cure or harden. At this point we are ready for the burnout process.
In this step the encased wax model is put into the kiln to be burned out or vaporized to make room for the pouring of the molten metal. This is generally accomplished by placing the flask into the kiln which has been preheated to 300 degrees. The flask is positioned, sprue side down, using the following time and temperature table when raising the temperature level in each specified stage.
TABLE ______________________________________ BURNOUT STAGES AND TEMPERATURES Flasks up to 21/2 in. .times. 21/2 in. Flasks up to 31/2 .times. 4 in. ______________________________________ 1st stage 1 hour at 300 degrees 1 hour at 300 degrees 2nd stage 1 hour at 600 degrees 2 hours at 600 degrees 3rd stage 1 hour at 1,100 degrees 2 hours at 1,100 degrees 4th stage 2 hours at 1,350 degrees 2-3 hours at 1,350 deg. 5th stage 1 hour. Reduce to 1 hour. Reduce to proper casting temperature proper casting temperature ______________________________________ Temperature not to be raised above 1,375 degrees
The burnout cycle should not be started unless the full process is to be completed. If a partially burned out flask is re-heated, it generally results in cracked investment and a ruined mold. After the prescribed time has elapsed, the flask is allowed to cool until it has reached the listed temperature for the metal to be used. This casting temperature is fairly critical.
______________________________________ INVESTMENT CASTING TEMPERATURE ______________________________________ Gold Thin, lacy jewelry articles 1,050 to 1,100 degrees Thick, heavy jewelry articles 800 to 900 degrees Sterling silver 750 to 850 degrees Bronze 900 to 1000 degrees Brass 750 to 800 degrees Pewter The flask should be cooled to room temperature (72 degrees) ______________________________________
Casting is the final step wherein the molten metal forced or drawn into the mold by one of the two methods-centrifugal or vacuum.
The centrifugal casting method utilizes a centrifugal casting machine to hurl the molten metal into the mold. To operate the centrifuge casting device, the casting crucible is first filled with metal and heated to melt the metal. The centrifuge is then activated to provide a constant force flow to force the metal into the mold.
Smaller production custom designed jewelry is generally cast using a conventional centrifugal arm casting machine powered by a spring driven motor. The conventional centrifugal casting machine has a rotating arm attached to the drive shaft of the motor, forming a first segment having a casting crucible mounted proximate its end; and a second segment with a movable counterbalancing weight attached near the other end to provide for balanced rotation. A good centrifugal machine will have a broken arm, which straightens out when the centrifuge reaches the desired force level so that the molten metal will not slide out the side of the crucible before the rotating arm develops sufficient speed to create a centrifugal force strong enough to force the molten metal directly into the flask. Examples of centrifugal broken-arm spring activated casting machines, Models CG2T, and CG4, produced under the trademark dixon.RTM.; and Model CA-1032 produced under the trademark VIGOR.RTM.; and Model Nos. 164-200 and 164-210 produced by Kerr. An example of an electrically driven motorized centrifugal caster is Model No. 160-040 produced by Rey.
The casting crucible has an open top with a pour spout aligned to pour molten metal into a casting flask having its pour opening in alignment with the axial length of the first arm.
The centrifugal casting machine must be balanced before each cast. This balancing step must be carried out before each burnout since there is little time from burnout to cast. To balance a centrifugal casting machine, the bent portion of the casting arm is straightened and wedged with a paper match. The invested flask, shield, crucible are set into place aligning the pour opening of the crucible with the opening of the mold. The metal that is to be used for the casting is placed into the crucible, and the center pivot thumb screw is loosened to let the arm balance at this point. Using the counterbalancing weight on the opposite end of the arm, the arm is brought into balance and secured to the shaft by tightening the thumb screw.
To commence centrifugal casting, the centrifuge spring drive motor is wound by turning the machine arm clockwise three or four full turns. When the required number of turns is reached, the arm is locked in place. The casting crucible is loaded with the premeasured amount of casting metal. The metal is then heated to the melting point with a gas torch. At the point where the casting metal becomes fluid and rolling, the hot burned out flask is removed from the kiln and mounted ahead of the crucible flush to the flask. Again the metal is heated, playing the flame until the metal melts and balls and rolls once more. A flux, such as borax, is generally sprinkled with the melting metal during heating to minimize oxidation. Strempel, U.S. Pat. No. 4,027 utilizes an inert gas delivery system associated with the centrifugal casting arm to cover the molten metal to prevent oxidation. Christian, U.S. Pat. No. 3,402,755 utilizes a thin metalizing layer to cover the mold before casting to prevent oxidation. At the point where the molten metal rolls freely, the centrifugal arm is released and spun by the drive motor.
The arm is allowed to stop, and the flask removed and submerged into a cool bucket of water. The investment will break up from the rapid temperature change and the casting will be released. In some cases a little push is needed.
After the metal cools, the investment is broken away from the finished casting. Small adhering pieces are then brushed away with water, or jetted away under a steam jet cleaner. To remove stubborn investment, a sand blaster can also be used.
To remove oxides and adhering glazed investment, the casting may be pickled with sulfuric acid or other pickling agents.
Although this centrifugal casting method is a significant improvement from a gravity flow pour method (wherein molten metal is simply poured into the mold), gas pockets can form within the mold so that its fine features are lost. The trapped gas in the mold can result in turbulence in the metal, as well as oxidation of the metal, causing porosity in the cast metal product.
To avoid the problem of trapped gas, vacuum assist casting was developed. Typically, vacuum assist casting first involves investing the casting flask mold under vacuum or vibration in a similar manner outlined above for centrifugal casting to eliminate air bubbles in the investment. The casting flask is then mounted on a vacuum table which creates a vacuum at one end of the flask to draw air and gases from the mold. Molten metal is then poured into the casting flask to gravity fill the mold. Although the gas pockets are removed, the molten metal is not forced into the mold by a constant flow force in excess of gravity. Consequently the mold may not properly be filled. Examples of combination investment and vacuum assist casting devices are: Model CCS-1752 Cal-Cast Invest and Cast; Model CCS-3000 Maxi-Vac Casting Unit; Model HY705-035 Rio Grande Vacuum Investing and Casting Machine; and Model 17-016 Pro-Craft.TM. Vacuum Machine for Investing and Casting.
One present solution to eliminate these problems is to employ a centrifuge entirely encased within a vacuum; see Sing, U.S. Pat. No. 4,781,237 as an example of a rotary casting apparatus employing a vacuum. However, these apparatus require expensive induction heating elements, and vacuum handling equipment operating enclosed entirely within a vacuum. This specialized equipment is not suitable for smaller production operations, which cannot carry the cost of the equipment. Nor do these devices apply a vacuum in a manner to assist the centrifugal force in forcing molten metals into the mold. Consequently, although these methods generally result in quality castings, porosity can develop.
Cited for general interest in Rohrberg, U.S. Pat. No. 3,865,173, which discloses a continuous gas welding system
Applicant's method and apparatus described below provides an inexpensive combination centrifugally fed vacuum assist method combining the advantages of both methods.